1. Field of the Invention
This invention relates to multiplexing techniques for obtaining status of a plurality of switch devices disposed along and connected to a single wire bus and more particularly in a preferred embodiment to systems for generating visual displays at a remote instrument panel indicative of the status of a series of switch sensors individually connected to indicating and transducing type switches disposed throughout a motor vehicle, the switch sensors being separately multiplexed onto a single wire, power and signal bus routed near the switches.
2. Description of the Prior Art
Electronic status display systems in vehicles are rapidly becoming commonplace. These systems provide status and display data communications between intelligent or smart modules. Display information comes from many functional systems in the vehicle, such as suspension status, anti-lock status and diagnostics modes. Usually there is a sensor associated with each system. Intermodule communications allow for rapid and accurate diagnosis of functional system failures. Interactive running diagnostics often provides early warning to the driver to get to a repair station. Also, prior to putting the vehicle into traffic, the status of a variety of functional systems can be checked using the electronic status display systems.
In the prior art, an electrical load monitoring system for monitoring the operating condition of various automobile electrical devices is described in U.S. Pat. No. 4,574,266 issued on Mar. 4, 1986 to R. J. Valentine. There, a load switching circuit is coupled to a common voltage line via a drive line input. A load monitoring circuit located remotely from the load switching circuit is also connected to the drive line input of the load switching circuit. The load monitoring circuit provides a test current to the load switching circuit and thereafter detects a response from the switching circuit. The test current is monitored to see whether it raises the drive line to the test current's supply voltage or whether the test current is shunted to a common voltage line by means of a MOSFET detector. The microprocessor will then indicate a load failure if the load has opened. In such a system, a separate power line and a drive line are provided to connect the various load switching circuits that are to be monitored. This arrangement appears to reduce somewhat the number of interconnections between an electrical load and a load status indicator, even though a circuit remote from the load switching circuit is needed for generating a test current.
Still another arrangement for monitoring status of a variety of vehicle functions over a single bus are described in U.S. Pat. No. 4,584,487 which issued Apr. 22, 1986, to Hesse et al. There, component modules communicate with a transmitter/receiver (T/R) module during time intervals on the bus. The T/R module codes and decodes this data and transmits it over a data communications link to a number of distribution units each having the capability of controlling a number of vehicle devices and providing status information to a microprocessor. This arrangement provides both status and control information for a plurality of vehicle components. But, this arrangement requires power lines, as well as the common bus line.